GENETICS AND RACE VARIABILITY OF THE LUCERNE - COLLETOTRICHUM-TRIFOLII PATHOSYSTEM IN AUSTRALIA

The Australian-bred lucerne cultivars, Trifecta and Sequel, were found to possess useful levels of resistance to both Colletotrichum trifoli i races 1 and 2. Race 2 has only been previously observed in the Unite d States and surveys did not reveal its presence in Australia. Multilo cus fingerprinting using random amplified polymorphic DNA (RAPDs) anal ysis revealed low diversity (<10% dissimilarity) within Australian C. trifolii collections, and between the Australian race 1 isolates and a US race 2 isolate. Studies on the inheritance of resistance to C. tri folii race 1 in individual clones from Trifecta and Sequel revealed th e presence of 2 different genetic mechanisms. One inheritance was for resistance as a recessive trait, and the other indicated that resistan ce was dominant. The recessive system has never been previously report ed, whereas in the US, 2 completely dominant and independent tetrasomi c genes Anl and Ant have been reported to condition C. trifolii resist ance. It was not possible to fit the observed segregations from our st udies to a single-gene model. In contrast to US studies, clones of cv. Sequel exhibiting the recessive resistance reacted differently to spr ay and stem injection with C. trifolii inoculum, being resistant to th e former and susceptible to the latter, providing additional evidence for the presence of a different genetic mechanism conditioning resista nce to those previously reported in the US. As C. trifolii is one of t he most serious diseases of lucerne worldwide, the future development of molecular markers closely linked to the dominant and recessive resi stances identified in these studies, and the relationships between the se resistances and Anl and Ans as determined by genetic mapping, appea r to be useful areas of future study.